Good job on your part, finding the problem.....
You don't know how many problems I have seen, caused by aftermarket, remote start/alarm installations.........seen it all......wires twisted together with tape, instead of being soldered or crimped.......have seen the quick connectors used that sometimes destroy the wiring they are attaching to......
But for the life of me, it amazes me that I see a lot of alarm systems put in GM cars, that already have alarm systems....
It was always my nightmare to drop a hush panel on the driver's side, and see a module come down, with a wiring harness poorly attached to the OEM wiring......I always told the customer, his/her best bet was to remove the system completely and repair all the connections....